The release of CO2 from combustion of carbonaceous fuels, and most specifically fossil fuels is of great concern due to the greenhouse effect of CO2 in the atmosphere. One approach to obtain reduction of CO2 emission into the atmosphere is CO2 capture from the exhaust gases from combustion of carbonaceous fuels and safe deposition of the captured CO2. The last decade or so a plurality of solutions for CO2 capture have been suggested.
The technologies proposed for CO2 capture may be categorized in three main groups:                1. CO2 absorption—where CO2 is reversibly absorbed from the exhaust gas to leave a CO2 lean exhaust gas and the absorbent is regenerated to give CO2 that is treated further and deposited.        2. Fuel conversion—where hydrocarbon fuels are converted (reformed) to hydrogen and CO2. CO2 is separated from the hydrogen and deposited safely whereas the hydrogen is used as fuel.        3. Oxyfuel—where the carbonaceous fuel is combusted in the presence of oxygen that has been separated from air. Substituting oxygen for air leaves an exhaust gas mainly comprising CO2 and steam which may be separated by cooling and flashing.        
WO 2004/001301 A (SARGAS AS) 31 Dec. 2003, describes a plant where carbonaceous fuel is com busted under an elevated pressure, where the combustion gases are cooled inside the combustion chamber by generation of steam in steam tubes in the combustion chamber, and where CO2 is separated from the combustion gas by absorption/desorption to give a lean combustion gas and CO2 for deposition, and where the lean combustion gas thereafter is expanded over a gas turbine.
WO 2006/107209 A (SARGAS AS) 12 Oct 2006 describes a coal fired pressurized fluidized bed combustion plant including improvements in the fuel injection and exhaust gas pre-treatment.
Combustion of the carbonaceous fuel under elevated pressure and cooling of the pressurized combustion gases from the combustion chamber reduces the volume of the flue gas, relative to similar amounts of flue gas at atmospheric pressure. Additionally, the elevated pressure and cooling of the combustion process makes a substantially stoichiometric combustion possible. A substantially stoichiometric combustion giving a residual content of oxygen of <5% by volume, such as <4% by volume or <3% by volume, reduces the mass flow of air required for a specified power production. The elevated pressure in combination with the reduced mass flow of air results in a substantial reduction of the total volume of the exhaust gas to be treated. Additionally, this result in substantial increase in the concentration and partial pressure of CO2 in the flue gas, greatly simplifying the apparatus and reducing the energy required to capture CO2.
All methods and processes for CO2 capture are energy consuming. Substantial effort has therefore been put into development of less energy consuming methods and processes to reduce the loss of energy, often in the form of steam at relatively low temperature and pressure, and cooling water. Many approaches have been made to heat integrate several process steps to ascertain that heat produced at one stage is transferred to a heat demanding process. The goal for these approaches are to get more energy efficient methods, processes and plants for production of electrical power from carbonaceous fuels at the same time as CO2 is captured.
There is, however, still a huge demand for solutions improving the energy efficiency of power plants including CO2 capture. The aim of the present invention is to provide novel an improved solutions for heat integration for increasing the energy efficiency, i.e. maximise the output of useful energy as heat and/or electricity of a given amount of chemical energy as carbonaceous fuel.